Spectrometric instruments include a dispersion element, such as a diffraction grating, and a detector system. In one precision type the detector is a conventional solid state device comprising an array of individual pixel photodetectors in which pixel location represents wavelength detected. Such a detector typically is a photodiode array, a charge coupled device or a charge injection device. Other spectrometric instruments scan the dispersion element over a single photodetector, or use Fourier transform of an interferogram, but the concepts are the same in that wavelength is calibrated against a physical location, orientation or the like. Modern instruments include a computer that is receptive of spectral data from the detector to analyze and compare spectra.
With improvements in optics, detectors and computerization, there has evolved an ability to perform very precise measurements. An example is an absorption spectrophotometer or polychromator using chemometric mathematical analysis to measure octane number in gasolines. Differences in octane number are associated with subtle differences in near infrared (IR) absorption spectra. The very small changes in spectral characteristics cannot effectively be detected directly by personnel, and computerized automation is a necessity. It also is desirable for such spectral measurements to be effected continuously on line. Thus there is an interest in utilizing advanced spectrometry methods for analytical chemistry.
Calibrations are carried out typically with spectral measurements on standard chemicals of known composition or other properties similar to unknown samples to be tested. Chemometric models are built from these spectra using multivariate calibration methods such as Principal Component Regression (PCR) or Partial Least Squares (PLS). As in the case of gasoline octane, this may require a large number (e.g. 50-100) samples for suitable precision, and accuracy, and calibrations may need to be repeated frequently to account for instrumental drift. Such model buildup also requires close scrutiny and expertise.
Calibrations also are performed with lamps or transmission filters that provide certain spectral lines of known wavelength. As such sources are available only for a few wavelengths, a fringe pattern such as with a Fabry-Perot interferometer is utilized to calibrate across the desired spectral range. Correlating the known wavelength with a fringe pattern has been a challenge. A mathematical model for estimating wavelengths with a standard and an interferometer is taught in a text "Fiber Optics in Astronomy" Volume 3, ed. by Samuel C. Barden (Astro. Soc. of the Pacific, 1988), pages 218-223. These methods apparently have not been applied to the field of analytical chemistry.
A typical spectrophotometer is described in "A Photodiode Array Based Near-Infrared Spectrophotometer For The 600-1100 nm Wavelength Region", by D. M. Mayes & J. B. Callis, Applied Spectroscopy 43 (1), 27-32 1989) and "Laptop Chemistry: A Fiber-Optic, Field Portable, Near-Infrared Spectrometer" by M. Lysaght, J. Van Zee and J. B. Callis, Reviews of Scientific Instrum. 62 (2) 507-515 (1991). Articles concerned with design, self-scanning and performance of multichannel spectrophotometric detector systems are "Self-Scanned Photodiode Array: High Performance Operation in High Dispersion Astronomical Spectrophotometry" by S. S. Vogt, R. G. Tull and P. Kelton, Applied Optics 17, 574-592 (1978); and "Self-Scanned Photodiode Array: A Multichannel Spectrometric Detector" by Y. Talmi and R. W. Simpson, Applied Optics 19, 1401-1414 (1989).
A problem with high precision measurements is that instruments vary from each other, and each instrument varies or drifts with time. The problem is partly one of achieving and maintaining calibration. A more subtle aspect is that the instruments have intrinsic characteristics that are individual to each instrument and also vary with time. Intrinsic characteristics distort the data effected by the instrument, rendering comparisons inaccurate. Such an intrinsic characteristic is typified by the profile of spectral data representing a very narrow, sharp spectral line. Such a profile has an intrinsic shape and line width wider than the actual line, due to the fundamental optical design as well as diffraction effects and other imperfections in the optics and (to a lesser extent) electronics in the instrument. An ideal profile is symmetrical, close to gaussian, but an actual intrinsic profile, also known as "instrument profile", may not even be symmetrical.
Therefore, a primary object of the present invention is to provide a spectrometric instrument with a novel means for effecting standardized spectral data. A further object is to provide a novel method for standardizing spectrometric instruments, particularly instruments having an intrinsic characteristic that distorts data, more particularly instruments having a characteristic intrinsic profile for narrow spectral lines. Other objects are to provide a novel method and means for transforming all data of the instrument so that such data is standardized for comparison with any other such standardized spectral data.
Further objects are to provide an improved method and means for calibration of spectrometric instruments, and for incorporating the aforementioned standardizing into the calibration. Another object is to provide a novel high finesse etalon that may be utilized in the calibration. Another object is to provide an improved method of determining spectral peak location of an optical standard, particularly for effecting the calibration.